"He was going to be her second son, my third grandchild, and we just can't come to terms with what's happened."

EARLIER

Geoff Mcintyre, 67, was one of the first on the scene at the crash on Radstock Avenue, Woodville Park, and says he and fellow neighbours did the best they could to help the woman who was left trapped in the wreckage.

Woodville crashSource:adelaidenow

"We were wiping the blood away from her her head," he said.

"She was sort of hanging out the back of the back window and then the driver climbed over the driver's seat and jumped out from the rear window which had shattered and ran off - naked.

"We wanted to chase him but he was running pretty well considering what had happened."

The pregnant Woodville Park woman, 23, was left with critical injuries after the car in which she was a passenger crashed into a tree and Stobie pole about 7.15am on Friday.

SA Road Deaths 2012

She was rushed to the Royal Adelaide Hospital where she and her unborn baby both died.

The driver - a Woodville Park man, 31 - was found by police a short time later and arrested.

Another nearby neighbour said the woman and the man were in the vehicle for up to 30 minutes before ambulance and fire crews arrived on the scene.

"The man was calling out for water and swearing at people," he said.

"We did what we could to help them both and to our surprise - he climbed out of the car and ran off."

Residents said the car seemed to be travelling very fast before it slammed into a Stobie pole.

Despite both victims being added to the road toll, South Australian common law does not recognise the unborn baby as a person.

Attorney-General John Rau last night told The Advertiser that he was concerned about the issue.

"In common law, an individual does not exist until they have been `born alive'," Mr Rau said.

He added: "I have personally argued a case before the Supreme Court to extend the boundaries of this definition."

At the time of the unborn baby's death, the driver had been charged with causing serious injury by dangerous driving, leaving the scene of an accident and driving unlicensed.

With the mother's death the causing serious injury by dangerous driving charge was upgraded to causing death by dangerous driving.

Police said that only one death by dangerous driving charge would be laid over the death of the mother. The man was refused bail and will appear in the Port Adelaide Magistrates Court on Monday.

Superintendent Howard Davies said police were in the very early stages of the investigation, but would look to see whether alcohol and drugs were involved.